As a technique for sensitive trace gas detection, laser absorption spectroscopy is attractive because of its simplicity and insensitivity to the environmental conditions of the absorbing species. Tunable diode laser (TDL) spectrometers are particularly suited to high sensitivity studies, in part because they may be frequency modulated to reduce low frequency laser noise and electronic noise. A typical spectrometer includes a frequency tunable laser for generating a laser beam which is directed through a sample cell and onto an optical detector. The signal received at the optical detector is demodulated to obtain the absorbance signal.
Unfortunately, sensitivity is usually severely limited by the presence of interference fringes (etalon fringes) which appear as the narrow bandwidth laser is tuned through the range of the desired absorbance signal. See, for example, Carlisle et al., Quantum Noise-Limited FM Spectroscopy with a Lead-Salt Diode Laser, Applied Optics, Vol. 28, No. 13, pp. 2567-2576, Jul. 1, 1989. The interference fringes are attributable to laser frequency dependent interference between pairs of parallel optical surfaces through which the laser beam must pass as it propagates from the laser, through the sample cell, and to the optical detector. The fringes may result from laser transmission through individual optical elements, such as windows or lenses, or through air and vacuum paths separated by the surfaces of different system elements. The reflections causing the interference fringes are extremely difficult to eliminate completely even with high quality anti-reflection coatings and careful optical alignment. These fringes, even when very weak, can easily overwhelm the absorbance signal from the sample volume.
Attempts have been disclosed to reduce the undesirable effects of such interference fringes. For example, U.S. Pat. No. 4,934,816 to Silver et al. discloses a mechanical approach to reducing interference fringes. A piezoelectric transducer translationally vibrates an active optical element, such as a mirror, in the optical path of the tunable laser. The interference fringes are then averaged over time to near zero using a sawtooth (triangle) waveform to drive the piezoelectric transducer that vibrates the optical element, so that each spatial position of the optical element is sampled with equal probability.
The Silver et al. patent also discloses that the fringe averaging effect may be obtained with a detection apparatus employing a lock-in amplifier referenced to the system modulation frequency which is asynchronous with respect to the vibration frequency of the active optical element. The most important consideration in choosing the vibration frequency of the active element according to the patent is that the vibration frequency be asynchronous with other system frequencies, particularly the lock-in reference frequency, so that fringe averaging can occur. See also Silver and Stanton, Optical Interference Fringe Reduction in Laser Absorption Experiments, Applied Optics, Vol. 27, No. 10, pp. 1914-1916, May 15, 1988.
Unfortunately, fast data acquisition cannot be performed with the Silver et al. technique, since averaging for a period longer than the period of the vibration frequency is required. Thus, the Silver et al. patent discloses mechanical modulation of the interference fringe at a very low frequency (less than about 100 Hz). Accordingly, the signal must be integrated to average out the fringes because the fringe signal is translated into very low frequency oscillation at the output of the lock-in amplifier. Moreover, the detection bandwidth of the spectrometer is limited to a very small range.
Another mechanical type approach to reducing interference fringes is disclosed in U.S. Pat. No. 4,684,258 to Webster. The Webster patent discloses a Brewster plate spoiler in the laser path downstream of the sample cell and operated by a controller for oscillating the spoiler back and forth about one axis. The oscillating spoiler cyclically varies the optical path length and creates standing waves in a second cavity exterior to the laser's inside cavity. The patent also discloses that for fringe averaging, the Brewster plate is preferably driven by a triangular wave oscillation signal which is several times the frequency of the free spectral range. Unfortunately, such a mechanical approach to fringe reduction is relatively complex, difficult to precisely control, and may not be fully satisfactory for removing unwanted interference fringes.
Other techniques have also been disclosed for attempting to reduce interference fringes in laser absorption spectroscopy. For example, an article by Carlisle, et al., Quantum Noise-Limited FM Spectroscopy with a Lead-Salt Diode Laser, Applied Optics, Vol. 28, No. 13, pp. 2567-2576, Jul. 1, 1989 discusses a two-tone frequency modulation technique for laser spectroscopy. The technique includes modulating the laser simultaneously at two arbitrary but closely spaced frequencies, and monitoring the beat tone between these two frequencies as the laser carrier and associated sidebands are tuned through a desired absorption line.
In the Carlisle et al. system, a conventional lead-salt diode is driven by superposition of three electrical signals. The signals are a DC current to forward bias the laser diode above threshold, a 1 KHz current ramp to repetitively sweep the laser output frequency across an absorption line of interest, and a two-tone radio frequency signal at a predetermined frequency using two radio frequency synthesizers and a double-balanced mixer. The 1 KHz current ramp and DC bias are adjusted in amplitude so that the output frequency of the laser just sweeps across the absorption line of interest. A low-pass filter is disclosed for removing interference fringes without significantly affecting the two-tone signal. However, if the free spectral range linewidth of the interference fringe is larger than or approximately equal to the absorption linewidth, the suppression of the fringe is severely limited. In addition, baseline noise cannot be eliminated because of the lowpass characteristic of the technique. Further disadvantages of the Carlisle et al. approach include a requirement for current ramping, and the requirement for a relatively complicated linear phase filter for recovering the absorbance signal. The approach is also not applicable when line locking is needed.
Other techniques using multiple beams are also known in the art. An article by Gehrtz et al., Quantum-Limited Laser Frequency-Modulation Spectroscopy, J. Opt. Soc. Am. B, Vol. 2, No. 9, pp. 1510-1526, Sept. 1985, describes frequency modulation spectroscopy including the difficulties associated therewith. In particular, the article discloses reduction of the residual amplitude modulation of the tunable laser by, for example, techniques using double laser beams.
An article by Cassidy et al. entitled Harmonic Detection with Tunable Diode Lasers--Two-Tone Modulation, Applied Physics, B 29, 279-285 (1982), discloses two-tone modulation to improve the sensitivity of tunable diode laser absorption spectrometers. A sinusoidal jitter modulation is applied having a predetermined phase and frequency to simultaneously minimize the fringe signal and increase the harmonic absorbance signal. Unfortunately, the fringe reduction technique is essentially lowpass in nature.
An article by Reid et al. entitled Sensitivity Limits of a Tunable Diode Laser Spectrometer, with Application to the Detection of NO.sub.2 at the 100-ppt Level, Applied Optics, Vol. 19, No. 19, pp. 3349-3354 (October 1980) discloses that minima of interference fringes occur when the amplitude of the sinusoidal modulation of the tunable diode laser is exactly an integral number of the interference fringe spacings. The article discloses that jitter modulation of a symmetric sawtooth at a frequency of 300-500 Hz and an amplitude much smaller than the 3 KHz sinusoidal modulation used for the second harmonic detection can be used to sweep the diode laser wavelength back and forth over exactly one fringe. This sweep occurs many times during an integration period of a lock-in amplifier set to the second harmonic (6 KHz) of the sinusoidal modulation, and hence the fringes are electronically washed out. The small amplitude of the jitter modulation ensures that is has little effect on the NO.sub.2 second harmonic absorption line shape.
Another application for laser spectroscopy is for analyzing and controlling plasma processing reactors, such as used for semiconductor processing. Such applications will require long term and short term laser frequency stability not available with conventional free-running lasers. This is especially true for use in low pressure processing where Doppler limited absorption linewidths of tens of megahertz may be encountered. Such narrow linewidths are of the same order of magnitude as the jitter broadened linewidth of a free-running laser. Furthermore, the high level of electrical noise generated by typical high power RF and microwave plasma sources exacerbates free-running laser frequency drift.
One approach to laser line locking is to divide the laser beam into a probe beam which passes through the gas sample under study, and a reference beam which is directed through a reference cell filled with a stable volume of the gas species under study. Since the reference cell is absorbance stable, the absorbance signal in the reference beam can be used to stabilize the laser frequency using any number of feedback schemes. See for example, Saito et al., "Frequency Modulation Noise and Linewidth Reduction in a Semiconductor Laser by Means of Negative Frequency Feedback Technique," Appl. Phys. Lett. 46, 3-5 (1985). Unfortunately, when probing transient species which are difficult to generate and detect in the first place, this reference beam approach is not satisfactory or cost efficient, and often impossible to practically implement.
Another approach for performing laser absorption spectroscopy is described in "Dual-Modulation Laser Line-Locking Scheme", by Bomse, Applied Optics, Vol. 30, No. 21; Jul. 20, 1991. The Bomse article describes a modulation approach using two levels of modulation to obtain a pair of measurement and control signals. The pair of signals originates in the first level (high frequency) of modulation. Accordingly, this approach limits the choice of FM frequency to values less than one half the bandwidth of the optical detector since second harmonic detection is required. Furthermore, it prevents choosing the FM frequency to optimize the overall signal amplitude. The second level (low frequency) of modulation is used only to eliminate baseline drift appearing after the first level of modulation. The laser frequency swing for this level of modulation must also be chosen to match the absorbance signal linewidth.